Gravity separation is usually used to remove solids associated with the activated sludge process. A methodology has been developed to improve settling of solids by gravimetric selection. This methodology might also be applied to decrease membrane fouling in a membrane bioreactor (MBR) process or to decrease membrane diffuser fouling. There are currently three approaches to select for solids that settle well. The first is strategies within an activated sludge process to select for well settling solids such as by using aerobic and anoxic or anaerobic zones or selectors to improve settling. However, there is a mixed history with the use of these selectors and it does not always work.
The second method includes using shear/agitation in a reactor to select for granular solids that settle well. This selection is also accompanied with an increase in the overflow rate of sludge in the mainstream solid-liquid gravity separator. This selection process is often gradual and tedious, and, since the selector is associated with the mainstream process, it can result in problems associated with meeting permit requirements. In most cases, only a sequencing batch reactor process allows the flexibility to increase over time and modify the overflow rate.
The third method includes selecting and wasting the poor settling foam and entrapped solids, often by collecting and “surface wasting” the foam and solids at the surface of a reactor using “classifying selectors”. While this approach was originally intended to reduce foam, it also selectively washes out the solids that do not settle well, as these slow settling solids tend to accumulate near the surface in reactors. Hence, this method retains only the solids that settle well, thereby providing a method that may be useful in deselecting poor settling solids, but which may have limited use in selecting settling solids. In implementing this method the settling characteristics improvements are often inconsistent, as sometimes poor settling solids, if they are produced at rates in excess of, e.g., a classifier surface removal rate, are retained and remain in the sludge.
An unfulfilled need exists for a method and an apparatus for wastewater treatment that does not have the drawbacks of the methods currently used to select and separate solids from wastewater.